Document Type : Research
Authors
1
MSc Graduate of Water Resources Engineering, Faculty of Agriculture, Tarbiat Modares University, Tehran, Iran
2
Assistant Prof., Faculty of Agriculture, Tarbiat Modares University, Tehran, Iran.
3
Associate Prof., Department of Geography, Faculty of Literature and Humanities, Ferdowsi University, Mashhad, Iran.
Abstract
River flow pattern is subject to constant changes as a result of changes caused by natural factors and human activities both in the catchment and along the river course. A sound knowledge of the phenomena involved and their impacts on flow conditions can obviously play effective roles in minimizing flood-associated casualties and in improving the exploitation and operation of the river as a resource. In the Gheshlaq basin, agricultural and orchard developments along the river banks over the past three decades have escalated the density of different land uses to increase pressure on this resource. To carry out a site investigation into the current status of the land uses along the river, satellite images obtained from Landsat-8 sensor OLI/TRIS and Landsat-5 sensor TM were used in the Google Earth Engine system with a spatial resolution of 30 meters. Subsequently, the random forest classification method was employed to derive land use maps for the years 1990, 2000, 2010, and 2020 and the changes in each period were detected and explored. The biggest changes were found to belong to the third period (i.e., 2010-2020) when residential development had increased by 23.3%, rainfed agriculture by 57%, and irrigated agriculture by 52.7% while pastures decreased by 16.9%. The river bed, boundaries, and transverse span of the river were also studied to detect land use changes to find increases in residential development and encroachments in the study periods: indeed, 154.1% of the bed of the Gheshlagh river was found to have been encroached in the period 1990-2000, 60% in the period 2010–2000, and 31.3% in the period 2010-2020. Evidently, these increases warrant necessary measures to clear the occupied sites for reclamation. Another aspect of the study involved the determination of peak discharges in the river basin throughout the study years using the HEC-HMS software based on rainfall intensities with different return periods in the study sub-basins along with CN values extracted from the combined land use maps and soil hydrological group maps. The results showed that peak flood discharge increased over time such that the greatest change in peak discharge belonged to the third period (2010-2020). This drastic increase in peak flood discharge was attributed to the declining pasture use and the increasing agricultural and residential land uses that, in turn, increased runoff potential and, thereby, CN values.
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